Thousands of globular clusters more than five billion years old have been resolved in the Virgo cluster of galaxies by the Hubble Space Telescope, providing detailed information about their creation and existence.

Globular star clusters are dense pockets of hundreds of thousands of stars residing in all but the very faintest of galaxies, and contain some of the oldest surviving stars in the Universe. Studying globular clusters is therefore critical to understanding the early, intense star-forming episodes that define and sculpt galaxy formation.

Using Hubble, astronomers identified over 11,000 such clusters in the Virgo cluster, which itself is made up of over 2,000 galaxies. At a distance of 54 million light years, it is the nearest large galaxy cluster to Earth, and the astronomers were able to resolve individual star clusters in 100 of Virgo’s galaxies of different size, shape and brightness, even in faint dwarf galaxies, discerning them from the near and far stars that can often confuse such observations.

Four galactic members of the Virgo Cluster that are part of the Hubble survey of globular clusters in 100 of Virgo's galaxies. Image: ESA, NASA and E. Peng (Peking University, Beijing).

"It's hard to distinguish globular clusters from stars and galaxies using ground-based telescopes", says Eric Peng of Peking University in Beijing, and lead author of the Hubble study. "With Hubble we were able to identify and study about 90 percent of the globular clusters in all our observed fields. This was crucial for dwarf galaxies that have only a handful of star clusters".

The observations also reveal a trend in the distribution of star clusters within dwarf galaxies. Up to several dozen globular clusters were found to reside in most of the dwarf galaxies located within three million light years of the galaxy cluster’s centre, with fewer towards the outskirts. This was a somewhat higher than expected central population considering the low masses of the dwarfs they inhabited, suggesting that the efficiency of star cluster formation is strongly linked to the crowded nature of the dense central environment.

Indeed, the giant elliptical galaxy Messier 87 (M87) also resides within the magic three million light year boundary of the Virgo cluster’s centre. Astronomers have long known that it hosts a larger than predicted population of globular star clusters, but their origin has been a long standing mystery, with the most popular theories suggesting that many of the clusters were snatched from smaller galaxies that strayed too close.

"We found few or no globular clusters in galaxies within 130,000 light years from Messier 87, suggesting the giant galaxy stripped the smaller ones of their star clusters", says Peng. "These smaller galaxies are contributing to the buildup of Messier 87".

M87, home to several trillion stars, a supermassive black hole and 13,000 globular star clusters, is the dominant galaxy at the centre of the Virgo Cluster of galaxies. The jet of material seen emanating from the centre is a black hole-powered stream of material being ejected from the galaxy's core. Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)/P. Cote (Herzberg Institute of Astrophysics) and E. Baltz (Stanford University).

Evidence of M87's galactic cannibalism comes from an analysis of the globular clusters' compositions, which shows that there are three times as many globulars deficient in heavy elements, such as iron, than globulars rich in those elements. “This suggests that many of these 'metal-poor' star clusters may have been stolen from nearby dwarf galaxies, which also contain globulars deficient in heavy elements,” says Peng.

Peng also suggests that the high rate of star formation in the galaxy cluster’s centre may be driven by the gravitational collapse of dark matter near the cluster's centre. “Messier 87 sits at the centre of a large concentration of dark matter, and all of these globulars near the centre probably formed early in the history of the Virgo cluster," he says.

The smaller number of globular clusters in the dwarf galaxies sitting further away from Virgo's centre may be due to less robust stellar formation at more distal locations producing less massive star clusters that dissipated over time.

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